Static fluid seals and seal assemblies for ultrahigh pressure fluid contaminment

ABSTRACT

High-pressure static seals and pressure vessels with static seals for containing fluid at high pressures are shown and described. Embodiments of the invention allow a plug to be easily, manually inserted into and removed from the pressure vessel. A metallic ring in the seal is configured to expand under pressure to prevent an O-ring in the seal from being extruded into a gap between the plug and the vessel wall, but not to expand so much as to cause galling or similar damage when the seal moves with respect to the wall under elevated pressure.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to fluid seals, and in particular,to devices and systems for sealing fluids at very high pressures.

[0003] 2. Description of the Related Art

[0004] Sealing fluids at extremely high pressures, i.e., pressures inexcess of 15,000 psi, can be extremely difficult and complicated. FIG. 5illustrates a high pressure seal according to the prior art. In theillustrated example, a plug 10 is engaged with a cylinder wall 12 havinga circular mouth. An exterior surface 14 of the plug 10 is closelyconformed to an interior surface 16 of the cylinder wall 12. A circulargap 18 is formed between the plug 10 and the cylinder wall 12. Anannular recess 20 is formed in the external surface 14 of the plug 10 ata distal end 22 of the plug.

[0005] A metallic ring 24 with a triangular cross-section is positionedwithin the recess 20 with its right-most edge 26 abutting acomplementary angled wall 28 of the recess. The metallic ring 24 issized to slide with the plug 10 with respect to the internal surface 16of the cylinder wall 12 when the system is not pressurized. This allowsthe plug 10 to be inserted and removed from the cylinder wall 12 toprovide access to a cavity 30.

[0006] A polymeric backup ring 32 (sometimes referred to as a seal) ispositioned to the left of the metallic ring 24, as viewed in FIG. 5. Anangled, left-most edge 34 of the metallic ring 24 abuts a complementarytapered edge 36 on the polymeric backup ring 32.

[0007] An O-ring 38 is positioned on the side of the polymeric backupring 32 opposite the metallic ring 24. The O-ring 38 is large enough toextend from the recess 20 to the internal surface 16 of the cylinderwall 12. The O-ring 38 seals the cavity 30.

[0008] When a fluid in the cavity 30 is pressurized, the O-ring 38 isurged against the polymeric backup ring 32 which, in turn, is urgedagainst the metallic ring 24. The tapered edge 36 of the polymericbackup ring 32 presses against the angled, left-most edge 34 of themetallic ring 24, creating an upward force that urges the metallic ringagainst the internal surface 16 of the cylinder wall 12. In addition, asthe metallic ring 24 is urged toward the right, as viewed in FIG. 5, thewall 28 of the recess 20 also urges the right-most edge 26 of themetallic ring upward against the internal surface 16 of the cylinderwall 12. In addition, the pressurized fluid operates on the left-mostedge 34 of the metallic ring 24, adding to the force urging the metallicring against the internal surface 16 of the cylinder wall 12.

[0009] As a result of the combined forces described above, the metallicring 24 is urged against the internal surface 16 of the cylinder wall 12with a very high force. As a result, the force the metallic ring 24exerts on the internal surface 16 is so great that relative movementbetween the two galls and scratches one or both of the contactingsurfaces.

[0010] When the fluid in the cavity 30 is pressurized to extremely highpressures (i.e., over 15,000 psi), or more so when the fluid ispressurized to even greater pressures (e.g., over 75,000 psi or over100,000 psi), the cylinder wall 12 expands outward and the plug 10compresses toward the right as viewed in FIG. 5. The expansion andmovement of these parts results in relative movement between themetallic ring 24 and the internal surface 16 of the cylinder wall 12.Every time the pressure in the cavity 30 is cycled, the metallic ring 24expands and contracts, further galling and scratching either themetallic ring and/or the cylinder wall 12. Eventually, scratches orother damage allows fluid to escape from the cavity 30, ultimatelyresulting in seal failure. In addition, it has been recorded that aftera number of cycles, the metallic ring 24 can become lodged against thecylinder wall 12, requiring further repair and replacement of parts ofthe system.

[0011] Attempts have been made to coat the metallic ring 24 withmaterials that prevent or delay damage. It has been found, however, thatsuch coatings are only temporary and, ultimately, the metallic ring 24again fails, as described above.

BRIEF SUMMARY OF THE INVENTION

[0012] The present invention is directed toward seals and seal systemsfor use with high pressure fluid containment systems. Embodiments of theinvention allow a plug or other closure to be easily, manually engagedwith and disengaged from a pressure vessel, while affecting a fluid sealat extreme elevated pressures. Embodiments of the invention will notscratch or gall the seal or the internal surface of the pressure vessel,and will not become lodged within the pressure vessel, as were commonlyexperienced with prior art seals.

[0013] One embodiment of the present invention incorporates a metallicring having inner and outer surfaces. The inner surface is adapted to bereceived within a recess in a plug or other closure, and the outersurface is adapted to closely conform with an internal surface of amouth on a pressure vessel. The maximum unstressed diameter of themetallic ring is equal to or slightly less than the diameter of themouth to allow the closure to be manually inserted into and removed fromthe mouth when fluid in the pressure vessel is not pressurized. An edgeon the metallic ring is adapted to sealingly conform to a complementaryedge on the recess when the seal is subject to an elevated pressure. Themetallic ring is made from a material having a modulus of elasticitythat is sufficiently low such that, when the fluid is pressurized, thefluid pressure expands the metallic ring against the wall of thepressure vessel with a force sufficient to prevent extrusion of anO-ring. At the same time, however, the modulus of elasticity of thematerial of the metallic ring is small enough such that the forcebetween the metallic ring and the wall is insufficient to generate ashear load great enough to gall the metallic ring when the metal ringmoves with respect to the wall.

[0014] In another embodiment of the present invention, the sealincorporates a metallic ring having an inner surface, an outer surface,and an edge similar to those described above. In this embodiment,however, the metallic ring has a specific width that is selected toprovide a desired pressure area. The width corresponds to the portion ofthe outer surface that contacts the wall of the pressure vessel. In thepresent invention, the width is large enough such that, when the fluidis pressurized, the metallic ring expands against the wall of the mouthwith a force sufficient to prevent O-ring extrusion. At the same time,however, the width is small enough such that the force is insufficientto generate a shear load great enough to gall the metallic ring when themetallic ring moves with respect to the internal surface.

[0015] In another embodiment of the present invention, the metallic ringincorporates a first ring and a second ring. The first ring can beconfigured according to either of the above embodiments. The second ringis spaced apart from the first ring and is configured to retain at leastone O-ring in the space between the first and second rings. In somealternate embodiments of this invention, the first and second rings areconnected by an elongated neck of metallic material. The length andthickness of the neck are selected such that the mass of the second ringdoes not adversely affect the performance of the first ring.

[0016] The present invention is also directed toward pressure vesselsincorporating the above-described seals.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0017]FIG. 1 is a diametric cross section of a portion of a pressurevessel and a plug therefor incorporating a seal according to anembodiment of the present invention.

[0018]FIG. 2 is an enlarged cross section of the pressure vessel, plugand seal of FIG. 1, as defined in FIG. 1.

[0019]FIG. 3 is a cross-sectional view of a portion of a pressure vesseland plug, and a seal assembly according to an alternate embodiment ofthe present invention.

[0020]FIG. 4 is a cross-sectional view of a portion of a pressure vesseland plug, and a seal assembly according to yet another alternateembodiment of the present invention.

[0021]FIG. 5 is a cross-sectional view of a portion of a pressure vesseland plug, and a sealing assembly according to the prior art.

[0022]FIG. 6 is a graph illustrating the relationship between theoperable width of a metallic ring according to one embodiment of theinvention, on one axis, and the stress exerted on the vessel wall bysuch a ring, on the other axis.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention is directed toward seals and sealingsystems for high-pressure fluid containment vessels. Many specificdetails of certain embodiments of the invention are set forth in thefollowing description and illustrated in FIGS. 1-4 and 6 to provide athorough understanding of such embodiments. One skilled in the art,however, will understand that the present invention may have additionalembodiments, or may be practiced without several of the detailsdescribed in the following description.

[0024]FIG. 1 illustrates a plug 10 and a cylinder wall 12 for a vesselused to contain high-pressure fluids. The applicant notes that the sealof the present invention can be used in a variety of configurations, andthat the configuration illustrated in FIG. 1 is merely one examplethereof. Applicant believes, however, that the following description andenclosed figures are sufficiently specific to allow one of ordinaryskill in the art to appreciate variations that can be made from theillustrated embodiment.

[0025]FIG. 2 is an enlarged view of a portion of the plug 10 andcylinder wall 12 illustrated in FIG. 1. Near the distal end 22 of theplug 10, a complex annular recess 120 is formed in the perimeter of theplug. A distal end of the recess 120 is open into the cavity 30, while aproximal end of the recess terminates in a sidewall 128. A metallic ring124 is positioned against the sidewall 128 of the recess 120. In theillustrated embodiment, the metallic ring 124 is an aluminum-bronzematerial of unitary construction. The applicant appreciates that theexact material of the metallic ring 124 can be varied as will beappreciated by one of ordinary skill in the art after reviewing thefollowing disclosure. An outer surface 125 on the metallic ring 124 issized and shaped to closely conform to the internal surface 16 of thecylinder wall 12. In the illustrated embodiment, the outer surface 125of the metallic seal 124 contacts the internal surface 16, withoutexerting any significant force on the internal surface. As a result, themetallic seal 124 can be slid with the plug 10 into and out of thepressure vessel manually.

[0026] A right-most edge 126 of the metallic seal 124 is shaped toclosely conform to the sidewall 128 of the recess 120. The angle of thetwo surfaces can be close enough to vertical, as viewed in FIG. 2, toprevent the metallic ring 124 from sliding with respect to the plug 10when the pressure in the cavity 38 increases. The right-most edge 126and the sidewall 128 can be processed to sealingly mate when the fluidin the cavity 30 is under pressure.

[0027] A neck 129 projects from the metallic ring 124 in a directiontoward the extreme distal end 22 of the plug 10. The neck 129 isradially spaced from the outer surface 125 to allow a backup ring 132and an outer O-ring 133 to be positioned between the neck 129 and theinternal surface 16 of the cylinder wall 12. An inner surface 135 of themetallic ring 124 can be spaced apart from the recess 120 to allow aninner O-ring 138 to be positioned between the metallic ring 124 and therecess 120. The inventors appreciate that the metallic ring 124 can befabricated without the neck 129 without deviating from the spirit of theinvention.

[0028] A retention ring 140 can be positioned distally on the plug withrespect to the metallic ring 124, the backup ring 132, the outer O-ring133 and the inner O-ring 138, to retain each of these elements in theirdesired configuration. A locking ring 142 can be retained within alocking channel 144 to prevent all of the above seal elements frommoving out of the recess 120 when the plug 10 is withdrawn from thecylinder wall 12.

[0029] The material selected for the metallic ring 124 has a modulus ofelasticity sufficiently low such that, when the pressure in the cavity30 rises and the internal surface 16 of the cylinder wall 12 movesoutward, the metallic ring can expand due to the fluid pressure on theinner surface 135. The modulus of elasticity of the material of themetal ring 124 is low enough, however, such that the force exerted bythe metallic ring on the internal surface 16 is not sufficiently largeto damage either the metallic ring or the cylinder wall 12 when themetallic ring moves with respect to the cylinder wall. The maximummodulus of elasticity thus corresponds to a shear force sufficient togall the material of the metallic ring 124. So long as the shear forceis below this threshold, movement of the metallic ring 124 with respectto the internal surface 16 will not result in galling of the metallicring.

[0030] The low range of the modulus of elasticity is based on the amountof expansion required for the metallic ring 124 to maintain contact withthe internal surface 16 of the cylinder wall 12 as the cylinder wallexpands under pressure, and to exert a force on the wall sufficient toprevent O-ring extrusion. In the illustrated embodiment, the modulus ofelasticity of the material of the metallic ring 124 is between 5 millionand 30 million psi, inclusive. In particular, the material of themetallic ring 124 is bronze sold under the brand Ampco M-4. The modulusof elasticity of this material is roughly one-half that of stainlesssteel, which is what the illustrated cylinder wall 12 is made from. Inalternate embodiments, the metallic ring 124 can be made from AluminumBronze 632 or other suitable materials.

[0031] During pressurization of the fluid in the cavity 30, the metallicring 124 is forced against the sidewall 128 of the recess 120, and atthe same time, is forced against the internal surface 16 of the cylinderwall. The only paths through which pressurized fluid can leak arebetween the metallic ring 124 and the sidewall 128 of the recess, orbetween the metallic ring and the internal surface 16 of the cylinderwall. In the first instance, the inner O-ring 138 prevents the passageof pressurized fluid, and in the latter instance, the outer O-ring 133prevents the passage of pressurized fluid.

[0032] The inner and outer O-rings 138/133 effectively resist theincreasing pressure within the cavity 30 so long as there is no gapbetween the metallic ring 124 and both the internal surface 16 and thesidewall 128 of the recess 120. As the pressure in the cavity 30increases substantially, however, the internal surface 16, with thecylinder wall 12, expands away from the plug 10. As a result, the sizeof the gap 18 between the plug 10 and the cylinder wall 12 increases.Rigid seals according to the prior art would allow the O-rings to beextruded into the increasing gap and, once the pressure decreases,become pinched and cut between the compressing cylinder wall and theplug. Eventually, the O-ring would fail as a result of the repeated cutscaused by the cycling of the pressure in the cavity 30. On the otherhand, highly elastic rings would expand with too much force, resultingin galling as the materials expand and contract, as discussed above.

[0033] Embodiments of the present invention prevent the O-ring frombeing cut, as the metallic ring 124 expands with the cylinder wall 12with a sufficient force to prevent the O-ring from being extrudedbetween the metallic ring 24 and the internal surface 16. The metallicring 124 remains in constant contact with the internal surface 16 of thecylinder wall 12. At the same time, however, the metallic ring 124exerts a force on the internal surface 16 that is not strong enough togall the material.

[0034] One experimental example is illustrated in FIG. 6. The optimalrange for the length (dimension “a” in FIGS. 3 and 4) for an aluminumbronze metallic ring having a diameter of approximately 6.0 inches isbetween 0.06 inches and 0.20 inches. Although a larger length wouldwork, the longer the length the greater the wear between the parts fordifferent materials and for rings of different diameters, this range mayvary, as one of ordinary skill in the art would appreciate afterreviewing this disclosure.

[0035] Embodiments of the present invention have numerous advantagesover the prior art. For example, embodiments of the seal according tothe present invention minimize or eliminate fretting and galling of themetallic ring and with the pressure vessel bore. In addition, becausethere is no fretting or galling, and because the metallic ring will notlodge against the cylinder wall, embodiments of the present inventionallow the plug to be manually, easily inserted into and removed from thepressure vessel.

[0036] Embodiments of the present invention provide a thorough fluidseal at extremely high pressures, while at the same time exerting littleor no force on the cylinder wall after the pressure has been removed.The metallic ring is able to slide axially along the cylinder wallwithout damaging the cylinder wall or being itself damaged thereby. As aresult, the useful life of the metallic ring of the present invention issubstantially longer and provides substantially more benefits during itsuseful life than seals of the prior art.

[0037]FIG. 3 illustrates one possible alternate embodiment of thepresent invention. The plug 10 of FIG. 3 has a complex recess 220 at itsdistal end 222. Within the recess 220, a metallic ring 224 ispositioned, along with inner and outer O-rings 238/233, and a backupring 232. The unitary metallic ring 224 is shaped to retain the innerand outer O-rings 238/233 and the backup ring 232 in their properrespective positions during use as well as during insertion and removal.

[0038] The metallic ring 224 of the present embodiment has a sealingportion 244 and a retention portion 246 spaced from the sealing portion.A neck 229 extends between the sealing portion 244 and the retentionportion 246. The sealing portion has a width “a” and a thickness “b”.The width “a” is designed to create a desired expansion force, asexplained above. The greater the width “a”, the greater the outwardforce exerted by the pressurized fluid against the sealing portion 244of the metallic ring 224. It is notable that the pressurized fluidexerts a force on opposing sides of the neck 229, and consequently doesnot generate any resultant outward force against the sealing portion ofthe metallic ring. Thus, the width “a” exclusively affects the force thesealing portion 244 of the metallic ring 224 exerts on the cylinder wall12.

[0039] The neck 229 is sufficiently thin to not significantly adverselyaffect the expansion of the sealing portion 244 of the metallic ring224. As a result, any resistance or expansion caused by the retentionportion 246 is not transferred through the neck 229 to the sealingportion 244. Likewise, any expansion by the sealing portion 244 is notresisted to any significant extent by the neck 229.

[0040] The one-piece metallic ring 224 of this embodiment allows for asimpler assembly due to the use of fewer structural elements. At thesame time, as discussed immediately above, the metallic ring 224 of thisembodiment can provide all of the advantages discussed in connectionwith the first embodiment of the present invention.

[0041]FIG. 4 illustrates yet another embodiment of the presentinvention. In FIG. 4, the metallic ring 324 has a retention portion 346.The retention portion 346, however, is configured to retain only one ofthe O-rings, as opposed to both O-rings as illustrated in FIG. 3. It isappreciated that the retention portion 346 could similarly be configuredto retain only the outer-most O-ring as opposed to the inner-most O-ringas illustrated in FIG. 4.

[0042] These and other variations can be made, as one with ordinaryskill in the art would appreciate based on reviewing the abovedisclosure and the enclosed figures. Therefore, it will be appreciatethat, although specific embodiments of the invention have been describedherein for purpose of illustration, various modifications may be madewithout deviating from the spirit and scope of the invention.Accordingly, the invention is not limited except by the appended claims.

1. A static seal for a pressure vessel designed to retain a fluidpressurized to at least 15,000 psi, the pressure vessel having a wallwith an internal surface and terminating at an access location in amouth, and having a plug removably engaged with the pressure vessel toprovide access to a cavity inside the pressure vessel, the plug havingan engagement portion that is closely conformed to the internal surfaceof the mouth, the engagement portion having a recess therein, an O-ringbeing positioned within the recess, the seal comprising: a metallic ringhaving an inner surface adapted to be received within the recess in theengagement portion of the plug, an outer surface adapted to closelyconform to the internal surface of the mouth, the metallic ring having amaximum unstressed diameter at the outer surface not greater than adiameter of the mouth such that the plug and seal can be easily,manually inserted into and removed from the mouth when the fluid is notpressurized, and an edge adapted to conform to a complementary edge onthe recess, and wherein the material of the metallic ring has a modulusof elasticity low enough such that, when the fluid is pressurized, themetallic ring expands against the wall of the mouth with a forcesufficient to prevent the O-ring from extruding between the metallicring and the wall, but small enough such that the force is insufficientto generate a shear force great enough to gall the metallic ring whenthe metallic ring moves with respect to the internal surface.
 2. Thestatic seal of claim 1 wherein the inner surface is spaced apart fromthe recess by an amount sufficient to allow an O-ring to be positionedtherebetween to facilitate sealing the fluid.
 3. The static seal ofclaim 1 wherein the metallic ring further comprises a side surfaceadapted to closely conform to an abutting side of the recess such that,when the fluid is pressurized, the side surface of the metallic ringdoes not move with respect to the side of the recess.
 4. The static sealof claim 1 wherein the metallic ring further comprises a side surfaceadapted to closely conform to an abutting side of the recess such that,when the fluid is pressurized, the side surface of the metallic ringdoes not move with respect to the side of the recess, and wherein theinner surface is spaced apart from the recess by an amount sufficient toallow an O-ring to be positioned therebetween to facilitate sealing thefluid.
 5. The static seal of claim 1 wherein the metallic ring furthercomprises a side surface adapted to closely conform to an abutting sideof the recess, the side surface approaching the internal surface at anacute angle, such that when the fluid is pressurized, the side surfaceof the metallic ring does not move with respect to the side of therecess.
 6. The static seal of claim 1 wherein the metallic ring furthercomprises a first side surface and an axially opposing second sidesurface, the first side surface being adapted to closely conform to anabutting side of the recess such that, when the fluid is pressurized,the side surface of the metallic ring does not move with respect to theside of the recess, and wherein the second side surface comprises a neckprojecting axially in a direction away from the side of the recess, theneck being spaced apart from the internal surface such that an O-ringcan be retained between the neck and the internal surface to facilitatesealing the fluid.
 7. The static seal of claim 1 wherein the metallicring further comprises a first side surface and an axially opposingsecond side surface, the first side surface being adapted to closelyconform to an abutting side of the recess such that, when the fluid ispressurized, the side surface of the metallic ring does not move withrespect to the side of the recess, and wherein the second side surfacescomprises a neck projecting axially in a direction away from the side ofthe recess, the neck being spaced apart from the internal surface suchthat an O-ring can be retained between the neck and the internal surfaceto facilitate sealing the fluid at low pressures, and the inner surfaceand neck being spaced apart from the recess by an amount sufficient toallow an O-ring to be positioned therebetween to facilitate sealing thefluid.
 8. The static seal of claim 1 wherein the metallic ring furthercomprises a side surface adapted to closely conform to an abutting sideof the recess such that, when the fluid is pressurized, the side surfaceof the metallic ring does not move with respect to the side of therecess, wherein the inner surface is spaced apart from the recess by anamount sufficient to allow an O-ring to be positioned therebetween tofacilitate sealing the fluid, and wherein a corner between the innersurface and the side surface is flared to intersect the side of therecess at a substantially perpendicular angle.
 9. The static seal ofclaim 1 wherein the metallic ring further comprises a first side surfaceand an axially opposing second side surface, the first side surfacebeing adapted to closely conform to an abutting side of the recess suchthat, when the fluid is pressurized, the side surface of the metallicring does not move with respect to the side of the recess, the secondside surface comprising a neck projecting axially in a direction awayfrom the side of the recess, the neck being spaced apart from theinternal surface such that a first O-ring can be retained between theneck and the internal surface to facilitate sealing the fluid, and theinner surface and neck being spaced apart from the recess by an amountsufficient to allow a second O-ring to be positioned therebetween tofacilitate sealing the fluid, and wherein a comer between the innersurface and the side surface is flared to intersect the side of therecess at a substantially perpendicular angle.
 10. A pressure vesseldesigned to retain a fluid pressurized to at least 15,000 psi, thepressure vessel comprising: a wall with an internal surface andterminating at an access location in a circular mouth; a plug removablyengaged with the pressure vessel to provide access to a cavity insidethe pressure vessel, the plug having an engagement portion that isclosely conformed to the internal surface of the mouth, the engagementportion having a recess therein; an O-ring positioned within the recessto seal a gap between the plug and the wall; and a metallic ring havingan inner surface adapted to be received within the recess in theengagement portion of the plug, an outer surface adapted to closelyconform with the internal surface of the mouth, a maximum unstresseddiameter at the outer surface not greater than a diameter of the mouthsuch that the plug and seal can be easily, manually inserted into andremoved from the mouth when the fluid is not pressurized, and an edgeadapted to conform to a complementary edge on the recess when the sealis subject to an elevated pressure, and wherein the material of themetallic ring has a modulus of elasticity low enough such that, when thefluid is pressurized, the metallic ring expands against the wall of themouth with a force sufficient to prevent the O-ring from being extrudedinto the gap, but small enough such that the force between the metallicring and the internal surface is insufficient to generate a shear loadgreat enough to gall the metallic ring when the metallic ring moves withrespect to the internal surface.
 11. The pressure vessel of claim 10wherein the inner surface is spaced apart from the recess, and furthercomprising an O-ring positioned therebetween to facilitate sealing thefluid.
 12. The pressure vessel of claim 10 wherein the metallic ring hasa first side surface and an axially opposing second side surface, thefirst side surface being closely conformed to an abutting side of therecess such that, when the fluid is pressurized, the side surface of themetallic ring does not move with respect to the side of the recess, andwherein the second side surface comprises a neck projecting axially in adirection away from the side of the recess, the neck being spaced apartfrom the internal surface, and wherein the O-ring is positioned betweenthe neck and the internal surface to facilitate sealing the fluid. 13.The pressure vessel of claim 10 wherein the metallic ring has a firstside surface and an axially opposing second side surface, the first sidesurface being closely conformed to an abutting side of the recess suchthat, when the fluid is pressurized, the side surface of the metallicring does not move with respect to the side of the recess, and whereinthe second side surface comprises a neck projecting axially in adirection away from the side of the recess, the neck being spaced apartfrom the internal surface, and wherein the O-ring is positioned betweenthe neck and the internal surface to facilitate sealing the fluid. 14.The pressure vessel of claim 10 wherein the metallic ring has a firstside surface and an axially opposing second side surface, the first sidesurface being closely conformed to an abutting side of the recess suchthat, when the fluid is pressurized, the side surface of the metallicring does not move with respect to the side of the recess, and whereinthe second side surface comprises a neck projecting axially in adirection away from the side of the recess, the neck being spaced apartfrom the internal surface, and the inner surface and neck being spacedapart from the recess, wherein a first O-ring is positioned between theneck and the internal surface, and further comprising a second O-ringbetween the metallic ring and the recess.
 15. A static seal for apressure vessel designed to retain a fluid pressurized to at least15,000 psi, the pressure vessel having a wall with an internal surfaceand terminating at an access location in a circular mouth, and having aplug removably engaged with the pressure vessel to provide access to acavity inside the pressure vessel, the plug having an engagement portionthat is closely conformed to the internal surface of the mouth, theengagement portion having a recess therein, an O-ring being positionedin the recess, the seal comprising: a metallic ring having an innersurface adapted to be received within the recess in the engagementportion of the plug, an outer surface having a width adapted to closelyconform with the internal surface of the mouth, the metallic ring havinga maximum unstressed diameter not greater than a diameter of the mouthsuch that the plug and seal can be easily, manually inserted into andremoved from the mouth when the fluid is not pressurized, and an edgeadapted to closely conform to a complementary edge on the recess whenthe seal is subject to an elevated pressure, and wherein the width ofthe outer surface is large enough such that, when the fluid ispressurized, the metallic ring expands against the wall of the mouthwith a force sufficient to prevent the O-ring from extruding between theplug and the wall, but small enough such that the force is insufficientto generate a shear load great enough to gall the metallic ring when themetallic ring moves with respect to the internal surface.
 16. The staticseal of claim 15 wherein the width is greater than or equal to 0.060inches.
 17. The static seal of claim 15 wherein the width is between0.060 inches and 0.20 inches.
 18. The static seal of claim 15 whereinthe metallic ring further comprises a first side surface and an axiallyopposing second side surface, the first side surface being adapted toclosely conform to an abutting side of the recess such that, when thefluid is pressurized, the side surface of the metallic ring does notmove with respect to the side of the recess, and wherein the second sidesurface comprises a neck projecting axially in a direction away from theside of the recess, the neck being spaced apart from the internalsurface such that an O-ring can be retained between the neck and theinternal surface to facilitate sealing the fluid, the neck beingsufficiently thin to not significantly affect the elasticity of themetallic ring.
 19. A static seal for a pressure vessel designed toretain a fluid pressurized to at least 15,000 psi, the pressure vesselhaving a wall with an internal surface and terminating at an accesslocation in a circular mouth, and having a plug removably engaged withthe pressure vessel to provide access to a cavity inside the pressurevessel, the plug having an engagement portion that is closely conformedto the internal surface of the mouth, the engagement portion having arecess therein, an O-ring being positioned in the recess to seal a gapbetween the plug and the wall, the seal comprising: a first ring ofmetallic material having an inner surface adapted to be received withinthe recess in the engagement portion of the plug, an outer surfacehaving a width adapted to closely conform with the internal surface ofthe mouth, the first ring having a maximum unstressed diameter at theouter surface not more than a diameter of the mouth such that the plugand seal can be easily, manually inserted into and removed from themouth when the fluid is not pressurized, and an edge adapted to conformto a complementary edge on the recess when the seal is subject to anelevated pressure, a second ring spaced apart from the first ring forbeing positioned on the plug distally of the first ring the second ringhaving an outer surface with a maximum unstressed diameter less thanthat of the first ring, and an inner diameter adapted to be closelyreceived within the recess, such that the second ring can retain atleast one O-ring in the recess between the first and second rings; andwherein the width of the outer surface of the first ring is large enoughsuch that, when the fluid is pressurized, the metallic ring expandsagainst the wall of the mouth with a force sufficient to prevent theO-ring from being extruded into the gap, but small enough such that theforce is insufficient to generate a shear load great enough to gall themetallic ring when the metallic ring moves with respect to the internalsurface.
 20. The static seal of claim 19 wherein the first and secondrings collectively form a compound ring of unitary construction, thefirst and second rings being connected by a neck of material, the neckbeing sufficiently thin and long as to not significantly affect theelasticity ratio of the first ring.
 21. The static seal of claim 19wherein the first and second rings collectively form a compound ring ofunitary construction, the first and second rings being connected by aneck of material, the neck being sufficiently thin and long as to notsignificantly affect the elasticity ratio of the first ring, the innersurface of the first ring being spaced apart from the recess to allow anO-ring to be positioned between the first ring and the recess tofacilitate sealing, the second ring being adapted to retain the O-ringbetween the first ring and the recess.
 22. The static seal of claim 19wherein the first and second rings collectively form a compound ring ofunitary construction, the first and second rings being connected by aneck of material, the neck being sufficiently thin and long as to notsignificantly affect the elasticity ratio of the first ring, the innersurface of the first ring being spaced apart from the recess to allow afirst O-ring to be positioned between the first ring and the recess tofacilitate sealing, the second ring being adapted to retain the firstO-ring between the first ring and the recess and to retain a secondO-ring between the neck and the internal surface of the mouth.